1. Field of the Invention
The present invention relates to a GaN substrate which is used in a semiconductor element, and in which the defect density is low. The present invention also relates to a process for producing a GaN substrate which is used in a semiconductor element, and in which the defect density is low. The present invention further relates to a semiconductor element including a semiconductor laser device which uses a GaN substrate in which the defect density is low.
2. Description of the Related Art
S. Nakamura et al. (xe2x80x9cViolet InGaN/GaN/AlGaN-Based Laser Diodes Operable at 50xc2x0 C. with a Fundamental Transverse Mode,xe2x80x9d Japanese Journal of Applied Physics, vol. 38 (1999) L226-L229) disclose a short-wavelength semiconductor laser device which emits laser light in the 410 nm band.
This semiconductor laser device is formed as follows. First, a GaN substrate is formed by growing a first GaN layer on a sapphire substrate, selectively growing a second GaN layer by using a SiO2 mask, and removing the sapphire substrate. Then, an n-type GaN buffer layer, an n-type InGaN crack preventing layer, an AlGaN/n-type GaN modulation-doped superlattice cladding layer, an n-type GaN optical waveguide layer, an undoped InGaN/n-type InGaN multiple quantum well active layer, a p-type AlGaN carrier block layer, a p-type GaN optical waveguide layer, an AlGaN/p-type GaN modulation-doped superlattice cladding layer, and a p-type GaN contact layer are formed on the above GaN substrate. However, the defect density in the semiconductor laser device is still high, and therefore the semiconductor laser device is not reliable in the high output power range.
In addition, T. S. Zheleva et al. (xe2x80x9cPendeo-Epitaxy-A New Approach for Lateral Growth of Gallium Nitride Structures,xe2x80x9d MRS Fall Meeting, Boston, 1998, Extended Abstracts G3.38) report that a flat GaN layer can be formed by utilizing lateral growth of GaN. In the reported process, a first GaN layer is formed without a mask, and then stripe regions of the GaN layer are removed until a sapphire substrate is exposed. Then, a second GaN layer is grown on the exposed sapphire substrate so that the second GaN layer is grown in the lateral directions.
Further, S. Nakamura (xe2x80x9cThree Years of InGaN Quantum-well Lasers: Commercialization Already,xe2x80x9d SPIE Proceedings, Vol. 3628, 1999, pp.158-168) reports that an InGaN-based multiple quantum well semiconductor laser device can be produced by using the above process proposed by T. S. Zheleva et al. However, the semiconductor laser device produced by the process is reliable only when the semiconductor laser device operates with the output power of 5 mW or less. Therefore, it is necessary to further decrease the defect density.
Furthermore, Japanese Unexamined Patent Publication, No. 10 (1998)-312971 discloses a process for preventing occurrence of a defect, such as a crack, which is caused by differences in the thermal expansion and the lattice constant between a GaN compound semiconductor layer and a sapphire substrate crystal. In the process, regions of growth are confined by a mask, facet structures of the GaN compound semiconductor layer are formed by epitaxial growth, and then the facet structures are further grown so that the mask is completely covered, and finally the surface of the grown crystal of the GaN compound semiconductor layer is planarized. However, in this process, the entire base layer on which the above GaN compound semiconductor layer is grown is formed on a substrate, and the lattice-mismatch between the base layer and the substrate is great. Therefore, the GaN compound semiconductor layer is affected by the substrate, the crystal orientations of the GaN compound semiconductor layer grown in lateral directions vary, and it is difficult to planarize the surface of the GaN compound semiconductor layer. Further, even when the above process is repeated, differences arise in the orientations of the crystal faces, and it is therefore impossible to reduce the defect density to a practical level.
Moreover, Japanese Unexamined Patent Publication, No. 11 (1999)-312825 discloses a process for realizing a low-defect region in a GaN layer formed on a GaN base layer by lateral growth, where the GaN base layer is formed on a plurality of portions of a surface of a sapphire substrate. In addition, a dielectric film is formed on the GaN base layer so as to suppress vertical growth from the GaN base layer. However, in this process, the crystal axis is likely to incline due to the mismatch between the sapphire substrate and portions of the GaN layer which are laterally grown over the sapphire substrate, or stress generated in the vicinity of the boundary between the sapphire substrate and the portions of the GaN layer. Further, as mentioned in Japanese Unexamined Patent Publication No. 11 (1999)-312825, a cavity is formed between the sapphire substrate and the laterally grown portions of the GaN layer, and the formation of the cavity is uncontrollable.
In the GaN substrate disclosed in Japanese Journal of Applied Physics, vol. 38 (1999) L226-L229, the SiO2 film stops the dislocation which is caused by the lattice mismatch in the vicinity of the boundary between the GaN substrate and the GaN buffer layer, and extends in the thickness direction. In addition, the aforementioned second GaN layer is formed mainly by the lateral growth from a plurality of portions of the aforementioned first GaN layer which are exposed at a plurality of windows of the SiO2 mask. However, since the laterally grown portions of the second GaN layer coalesce in central portions of a plurality of regions which are located above the remaining SiO2 film of the SiO2 mask, defects tend to gather in the central portions of the plurality of regions above the remaining SiO2 film. In addition, dislocation is likely to extend in the thickness direction, and pass through the above plurality of windows, Therefore, only the above plurality of regions above the remaining SiO2 film other than their central portions are low-defect regions of the second GaN layer. Such low-defect regions each have a width about 4 micrometers. That is, the low-defect regions are very narrow, and the semiconductor laser devices having a stripe of a 2 xcexcm width must be formed in such narrow regions.
In addition, according to the processes disclosed in the Extended Abstracts G3.38 of the MRS Fall 1998 Meeting and the SPIE Proceedings, Vol. 3628, 1999, pp.158-168, defects also tend to gather in a plurality of regions in which laterally grown portions of the aforementioned second GaN layer coalesce, In addition, the dislocation is likely to extend in the thickness direction from the first GaN layer, which functions as a base of the growth of the second GaN layer. Therefore, the low-defect regions in the second GaN layer are very narrow, and the semiconductor laser devices having a stripe of a width of several micrometers must be formed in such narrow regions.
An object of the present invention is to provide a GaN substrate which is used in a semiconductor element, and in which the defect density is low in a wide region.
Another object of the present invention is to provide process for producing a GaN substrate which is used in a semiconductor element, and in which the defect density is low in a wide region.
Still another object of the present invention is to provide a semiconductor element which uses a GaN substrate in which the defect density is low in a wide region.
A further object of the present invention is to provide a semiconductor laser device which uses a GaN substrate in which the defect density is low in a wide region.
(1) According to the first aspect of the present invention, there is provided a GaN substrate comprising: a substrate; a first GaN layer being formed on the substrate and including a plurality of stripe portions which form at least one first groove between adjacent ones of the plurality of stripe portions; a second GaN layer formed over the substrate and the first GaN layer; a first preventing means, arranged at upper surfaces of the plurality of stripe portions, for preventing crystal growth of a GaN layer in the vertical up direction from the upper surfaces of the plurality of stripe portions; and a second preventing means, arranged at at least one bottom of the at least one first groove, for preventing crystal growth of a GaN layer in the vertical up direction from the at least one bottom.
The first GaN layer may be comprised of only said plurality of stripe portions. Alternatively, the first GaN layer may further comprise at least one bottom portion in the at least one first groove.
In the GaN substrate according to the first aspect of the present invention, the crystal growth of a GaN layer in the vertical up direction from the upper surfaces of the plurality of stripe portions of the first GaN layer is prevented by the first preventing means, and the crystal growth of a GaN layer in the vertical up direction from the at least one bottom of the at least one first groove formed between the plurality of stripe portions of the first GaN layer is prevented by the second preventing means. Therefore, in the initial stage of the crystal growth of the second GaN layer, the crystal grows only in the lateral directions. Thus, it is possible to prevent the dislocation which extends from a lower layer in the thickness direction, and occurs in the conventional GaN substrate. Consequently, the GaN substrate according to the first aspect of the present invention includes a wide, low-defect region.
Preferably, the GaN substrate according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv).
(i) The first preventing means may be realized by a dielectric film formed on the upper surfaces of the plurality of stripe portions.
In this case, the crystal growth of a GaN layer in the vertical up direction from the upper surfaces of the plurality of stripe portions of the first GaN layer can be effectively prevented.
(ii) The second preventing means may be realized by a dielectric film formed on the at least one bottom of the at least one first groove.
In this case, the crystal growth of a GaN layer in the vertical up direction from the at least one bottom of the at least one first groove formed between the plurality of stripe portions of the first GaN layer can be effectively prevented. Therefore, the crystal growth of a GaN layer from the exposed side walls of the plurality of stripe portions of the first GaN layer can be promoted, and no defect extends from the at least one bottom of the at least one first groove in the thickness direction. Further, when the composition and the quality of the dielectric film is appropriately controlled, it is possible to prevent deterioration of crystallinity due to the inclination of the crystal axis which is caused by the stress generated in the vicinity of the dielectric film, and the like.
The dielectric films used as the first and second preventing means may be made of an oxide such as silicon oxide, titanium oxide, zirconium oxide, and aluminum oxide, or a nitride such as silicon nitride, aluminum nitride, and titanium nitride, or an oxynitride such as silicon oxynitride and aluminum oxynitride. Alternatively, the dielectric films may be a multilayer film made of any combination of the above films.
(iii) The GaN substrate according to the first aspect of the present invention may further comprise a low-temperature GaN buffer layer arranged under the plurality of stripe portions.
In this case, the low-temperature GaN buffer layer contributes to reduction of crystal defects in the GaN layer formed on the low-temperature GaN buffer layer.
(iv) The GaN substrate according to the first aspect of the present invention may further comprise, between the substrate and the first GaN layer, a low-temperature GaN buffer layer formed on the substrate, a third GaN layer formed on the low-temperature GaN buffer layer, and a dielectric film being formed on the third GaN layer and realizing the second preventing means.
In this case, the crystal growth of a GaN layer in the vertical up direction from the bottom of the first groove formed between the plurality of stripe portions of the first GaN layer can be effectively prevented.
(v) In the GaN substrate having the additional feature (iv), at least one portion of the dielectric film which is not located under the plurality of stripe portions of the first GaN layer may be removed so as to form at least one second groove, and make at least one gap between at least one bottom of the at least one second groove and the second GaN layer.
In this case, the second GaN layer can be formed by only the lateral growth from the exposed side walls of the first GaN layer, and it is therefore possible to prevent occurrence of a defect which extend from the at least one bottom of the at least one second groove in the thickness direction.
(vi) Each of the at least one first groove may have a width of 20 micrometers or greater.
In this case, since low-defect regions are realized in the second GaN layer except for the portions in which the laterally grown GaN portions coalesce, the low-defect regions in the GaN substrate can have a width of about 10 micrometers.
(2) According to the second aspect of the present invention, there is provided a semiconductor element having at least one semiconductor layer formed on a GaN substrate according to the first aspect of the present invention.
Since the semiconductor element according to the second aspect of the present invention is formed by growing semiconductor layers on the GaN substrate according to the first aspect of the present invention, the characteristics and reliability of the semiconductor element can be improved.
Preferably, the semiconductor element according to the second aspect of the present invention may also have one or any possible combination of the aforementioned additional features (i) to (vi).
(3) According to the third aspect of the present invention, there is provided a semiconductor laser device having a plurality of semiconductor layers formed on a GaN substrate, wherein a current injection window having a width of 10 micrometers or greater is formed in the plurality of semiconductor layers, and the GaN substrate is according to the first aspect of the present invention.
Since the semiconductor laser device according to the third aspect of the present invention is formed on the GaN substrate which includes a wide low-defect region, and the width of the current injection window is 10 micrometers or greater, the semiconductor laser device according to the third aspect of the present invention is reliable even when the semiconductor laser device operates with high output power.
Preferably, the semiconductor laser device according to the second aspect of the present invention may also have one or any possible combination of the aforementioned additional features (i) to (vi).
(4) According to the fourth aspect of the present invention, there is provided a process for producing a GaN substrate, comprising the steps of: (a) forming a first GaN layer on a substrate; (b) arranging at an upper surface of the first GaN layer a first preventing means for preventing crystal growth of a GaN layer in the vertical up direction from the upper surface of the first GaN layer; (c) removing at least one stripe area of the first preventing means and the first GaN layer from an upper surface of the first preventing means to a partial or full thickness of the first GaN layer or a partial thickness of the substrate so as to form at least one groove; (d) arranging at at least one bottom of the at least one groove a second preventing means for preventing crystal growth of a GaN layer in the vertical up direction from the at least one bottom; and (e) forming a second GaN layer over the first GaN layer and the substrate.
In the process according to the fourth aspect of the present invention, the GaN crystal grows only in the lateral directions in the initial stage of the crystal growth of the second GaN layer. Therefore, low-defect regions are realized in the second GaN layer except for the portions in which the laterally grown GaN portions coalesce. That is, a GaN substrate which includes a wide low-defect region can be produced by the process according to the fourth aspect of the present invention.
(5) According to the fifth aspect of the present invention, there is provided a process for producing a GaN substrate, comprising the steps of: (a) forming a first GaN layer on a substrate; (b) arranging on a plurality of portions of an upper surface of the first GaN layer a first preventing layer which prevents crystal growth of a GaN layer in the vertical up direction from the plurality of portions of the upper surface of the first GaN layer; (c) forming a second GaN layer over the first GaN layer and the first preventing layer; (d) removing at least one first portion of the second GaN layer so that a plurality of second portions of the second GaN layer remain only on all or a portion of the first preventing layer, and at least one groove is formed between adjacent ones of the plurality of second portions of the second GaN layer; (e) arranging, on at least one bottom surface of the at least one groove and upper surfaces of the plurality of second portions of the second GaN layer, a second preventing layer which prevents crystal growth of a GaN layer in the vertical up direction from the at least one bottom surface and the upper surfaces of the plurality of second portions of the second GaN layer; and (f) growing a third GaN layer from side walls of the plurality of second portions of the second GaN layer until an upper surface of the third GaN layer is planarized.
In the process according to the fifth aspect of the present invention, the GaN crystal grows only in the lateral directions in the initial stage of the crystal growth of the third GaN layer. Therefore, low-defect regions are realized in the third GaN layer except for the portions in which the laterally grown GaN portions coalesce. That is, a GaN substrate which includes a wide low-defect region can be produced by the process according to the fifth aspect of the present invention.